The major purposes of this research project are: (1) to isolate and chemically characterize the murine thymus-leukemia (TL) antigens, (2) to determine the cellular and molecular mechanism of antigenic modulation of TL antigens and (3) to search for TL antigen analogues on the thymus and leukemia cells of other species. The biological significance of the TL antigens relates mainly to their highly restricted tissue distribution in normal (TL positive) mice and to the loss or masking of the TL antigens at a critical stage of thymocyte differentiation when it is presumed that precursor cells become immunocompetent T-cells. Our studies will attempt to elicit rabbit antisera specific for various TL phenotypes and to use these antisera to prepare immunoadsorbents for the isolation of the bulk quantities of antigen that are necessary to perform chemical studies. We plan also to use rabbit anti-beta 2 microglobulin antisera and anti-TL antisera to determine the polypeptide chain composition of the TL antigens and to study the complementary relationships between TL and H-2 antigens. The availability of rabbit anti-TL antisera will also facilitate the search for TL antigenic determinants in extra-thymic murine tissues and on the thymus and leukemia cells of other species. To further characterize the structural features of the TL antigens should provide insights into their role in T-cell differentiation and leukemogenesis. BIBLIOGRAPHIC REFERENCES: Esmon, N., and Little, J.R.: An Indirect Radioimmunoassay for Thymus Leukemia (TL) Antigens. J. Immunol. 117:911, 1976. Esmon, N., and Little, J.R.: Different Mechanisms for the Modulation of TL Antigens on Murine Lymphoid Cells. J. Immunol. 117:919, 1976.